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Tuesday, May 28, 2013

Have your multiverse and eat it

The recent results from the Planck mission have caused a flurry of activity among theoretical physicists, documented on the arXiv in an increasing amount of papers with updates on constraints on various cosmological models. Of particular interest is the question which models of inflation are favored by the data. Interestingly, the simplest potentials for the scalar field that causes inflation are ruled out or disfavored already. For a summary, see Jester’s post Planck about inflation.

Paul Steinhardt and collaborators have taken this as a reason to argue that the data actually hints at cyclic models.

The potentials for the inflaton field that are necessary to fit the Planck data are not simple in that they require finetuning, ie delicately adjusted parameters. The finetuning has to produce a suitably flat plateau in the potential, and a power law with coefficients of order one isn’t going to do this. If you’d random pick the potential, it would be very unlikely you’d get a suitably finetuned one.

This, Steinhardt et al argue, is a serious problem because the “inflationary paradigm” draws its justification from our universe being a “likely” outcome of quantum fluctuations that are blown up to produce the structures we see. If the potential, or the initial value of the scalar field, is unlikely, this erodes the basis of believing in the inflationary paradigm to begin with. In the paper this unlikeliness is quantified, and it is noted that the unlikeliness of the initial value of the scalar field can be recast as an unlikeliness of the potential. Then they go on to argue that cyclic models are preferable because in these cases natural parameter ranges for coefficients in the potential are still compatible with the data (they do not comment on how natural these models are in other respects). They then identify observables that could further solidify the case.

There are two gaps in this argument. The first gap is between “inflation” and “inflationary paradigm.”

Inflation is a model that describes very well the observations in our universe by using a familiar framework that makes use of quantum field theory and general relativity. The inflationary paradigm that they refer to (not an expression that is common in the scientific literature) adds requirements beyond the explanation of observation, that being the likeliness of the model.

To begin with, speaking about probabilities makes only sense if one has an ensemble. So to even refer to unlikeliness you have to believe in a distribution over set of possibilities, a multiverse. And for that you must have faith in your model, faith that extends beyond and before and beneath our universe, faith that the model holds outside everything we have ever observed, and that you can actually use it to make a statement about likeliness.

Besides this, the inflaton potential is normally not expected to be fundamental, but some effective limit a few orders of magnitude below the Planck scale. If you want to say anything about the probability of finding a particular potential, you would first have to know the fundamental degrees of freedom and the UV-completion of the theory. Just taking potentials and attempting to assign them a probability doesn’t make a lot of sense.

So talking about probabilities is already a bad starting position. From this starting position then Steinhardt et al argue that the inflationary paradigm says that we should find our universe to be likely. But by going from inflation to the inflationary paradigm, one is no longer talking about testing a model that explains observations. In their own words

That should be the last sentence of a scientific paper. Alas, there’s a next sentence, and it starts with “However…”

“However, this cannot be described as a success for the inflationary paradigm, since, according to inflationary reasoning, this particular class of models is highly unlikely to describe reality.”

Note the leap from “theory” to “paradigm”. (Let me not ask what “reality” means, I know it’s an unfair question.)

The second gap in the argument is that you could use it to rule out pretty much any model anybody has ever proposed.

In this earlier post I explained that all presently existing theories inevitably lead to a multiverse, a large space of possibilities. It’s just that this multiverse is more apparent in some approaches than in others.

The reason a multiverse is inevitable is that we always need something to specify a theory to begin with. Call it basic axioms or postulates. We need something to start with. And in the context of the theory you’re working with, that postulated basis is an uncaused cause: It was written down with the explicit purpose to explain observations. If you take away that purpose because you’ve misunderstood what science is all about, you are left with only mathematical consistency. And then, layer by layer, you are forced to include everything into your theory that is mathematically consistent. That’s what Tegmark called the “Mathematical Universe.”

Steinhardt et al’s elaboration about the possible shape of potentials is an example of this mathematical multiverse beneath the basis. They take away one postulate and replace it by a larger space of mathematical possibilities. Instead of postulating a specific (purpose bound) real-valued, differentiable, scalar function, they replace them with the space of all continuous functions (though they’re not too explicit on the requirements). But why stop there? Why not take the space of all functions and random pick one of these? Almost all functions on the real axis are discontinuous in infinitely many places, which is a fancy way of saying that the probability to get a continuous one upon random picking is zero. Look, I just ruled out both the “inflationary paradigm” and Steinhardt’s cyclic models without referring to any data at all.

To be fair however, Steinhardt et al are just fighting inflation with its own weapons. It is arguably true that the literature is full of arguments about naturalness and how inflation solves this or that philosophical conundrum. If you believe in the multiverse, or eternal inflation specifically, I think you should take the argument put forward in these papers seriously. For the rest of us, those who see inflation as a model with the purpose to describe observations in our universe, there’s no reason to make these leaps of faith. And that’s what they are - at least for now. One never knows what the data will bring.

58 comments:

Seems we have several problems of a similar sort, such as requiring a scalar with a plateau-potential for inflation, requiring an axion or something to solve the strong CP problem, requiring dark matter to account for various observations of motion & lensing on various scales, dark energy - lots of things that we put into models to fit remote observations but on which we have no theoretical handle and no experimental access.

Seems like we are ripe for a revolution in physics; especially since the only idea in town appears to be the multiverse which is as lame as the luminiferous aether.

Superb post Bee, v helpful to see this articulated clearly. Almost every recent popular book I have on cosmology contains a statement like: "Inflation solves several apparent coincidences beautifully, but requires some special conditions of its own". The plural always puzzles me because, as I undesrtand it, all that is required is a particular type of potential. Particular yes, but no less likely than many others, so it's the old conundrum of whether something is a coincidence or just an incidence!

"One never knows what the data will bring" "This is not the solution we are seeking," "if there is no precedent, the idea is wrong," "it contradicts accepted theory." Theorize what cannot be tested. The Emperor is both clothed and unclothed.

"To be fair however, Steinhardt et al are just fighting inflation with its own weapons."

You meant to say, "Inflationary paradigm", not "inflation", right?

The problem here of course is that the people who invented inflation are also the people pushing the inflationary paradigm. I think that most people who have really thought about it don't buy Linde's claim that inflation can just start by itself, without any special previous conditions -- obviously the second law of thermodynamics applies to the inflaton too, and inflation can only increase entropy, just like everything else. Most of us just think that inflation is a good theory of an important part of cosmic history, not some magical machine that can explain the initial conditions.

Well, yes, I should have written "inflationary paradigm", you are right. Alas, as you say, many people who have pushed inflation have done so by alluding to the inflationary paradigm. Unnecessarily, I think, when they should just have focused on predictions. That is to say, I have some sympathy for the argument Steinhardt et al are making, I just don't think it's a very convincing case for their model either. Best,

You are right of course that inflation happens at densities many times higher than what we have ever tested, employing processes at timescales many times faster and distances many times shorter than what we have ever tested. GR is expected to break down indeed, but that is at scales even higher than that. You write

"there is good reason to believe that this extrapolation will not work..."

There is good reason to believe that it does work, which is that it fits extremely well with our observations. Best,

Re Robert oldershaw's comment "Until the dark matter is definitively identified and some theory has anticipated that discovery, we only have questionable fashions based on selective analysis of minimal information".It sounds like he has confused dark matter with dark energy. The former simply involves removing the assumption that all gravitating matter interact with the electromagnetic force, hardly a 'fashion'.

"Yep, enigmatic - I wonder which one is the most enigmatic one ?http://en.wikipedia.org/wiki/List_of_unsolved_problems_in_physics"

At least half or more of those questions are answerable today. The problem is that the ones who are wrong are mostly the ones in the highest position of authority. They are the ones with the clout in media. (Just try writing something that is correct but contrary to the conventional wisdom of today and you'll see what I mean.)

As a boy growing up in the 50's Einstein was treated as a great genius but his ideas, especially GR, wasn't even mainstream then. I remember reading up to date encyclopedias (remember encyclopedias)where the aether was still treated with respect and GR was mentioned just in passing as a very controversial theory by Dr. Einstein.

Believe me, Wikipedia won't catch up to the cutting edge for another 40 years. It doesn't matter that it's publicly editable. People there are just generally stupid and will revert it. Not that different from the encylopedias of yore.

"Scientists did not expect dark matter and many fought against the concept until the evidence was quite strong. Some still fight against it (e.g., MOND)."

Right, it was not expected. But that doesn't mean that, once evidence for it exists, one should still doubt it. Dark matter is like gorillas before they were known in Europe: Gorillas were not expected, no theory predicted them, but once they were shown to exist, there was no reason to doubt them. Of course, one had to wait for good evidence, as there were many legends of strange beasts, not all of which actually existed.

In retrospect, I do think it strange that anyone should expect that all matter is non-dark and/or baryonic. However, that doesn't mean one should have expected it.

I probably was too flippant, but Wikipedia does not have my respect. All too often the articles in physics come off as seeming like the effort of mediocre people emulating in writing their idea of what a smart person would write. And in the areas of physics I'm familiar with, not nearly all the subjects in that article, they just seem completely wrongheaded. I won't go into any more detail than that.

Certainly the quality of Wikipedia articles varies enormously. Readers of German might appreciate this article about what are apparently hoaxes in Wikipedia which managed to survive for years in some cases: http://einestages.spiegel.de/s/tb/28578/wikipedia-hoaxes-schwindel-in-der-online-enzyklopaedie.html

More common are perhaps single sentences in an otherwise acceptable article which are wrong or misleading but also the pet project of some nerd (which becomes obvious when any changes are reversed within minutes).

I haven't read many physics articles in Wikipedia, but my impression is that some of the good ones are pitched at a level which is much too high for Wikipedia.

"It turns out Wikipedia is not actually the project of many, but the project of just a few. It is a widespread misconception that all users contribute mutually and democratically to the site and that Wikipedia is the product of "collective intelligence." The German Wikipedia has several hundred thousand registered users. But according to research carried out by sociologist Christian Stegbauer in Frankfurt, more than half of all users who register on the site never make changes to an article. Only 0.5 percent of all active users are responsible for nearly two-thirds of all editing. That's a group of not even 2,000 people."

That's from another article in Der Spiegel International:http://www.spiegel.de/international/germany/backstage-with-the-wikipedians-inner-workings-of-global-encyclopedia-better-than-a-soap-opera-a-690402.html

My feeling is Wikipedia was a good idea that pretty much went bad from within.

More than a half century ago I read through as much of the science encyclopedia that came with the Book of Knowledge general encyclopedia ( MY parents had bought the general encyclopedia instead of Britannica because I wanted the science encyclopedia. They could not afford Britannica. I read as much I had time for. Not all of it was correct and of course it was updated only once a year. Part of learning is learning when to check further. Who would have imagined at fingertip the coolest ever expanding library with almost always discussion of every morsel in it. Anyone can write a book or post to Wikipedia. As when I was a child whether it offers anything more valuable than it's caloric content as fuel depends on who, when and where in space time it is.

I'm sure that your comments say much more about you than they do about me.

We don't know what dark matter is. You don't, I don't, no-one else does. Nevertheless, you make the folliowing claim: If dark matter consists of WIMPs, then they must annihilate or decay at a rate which is detectable by current technology. On what is this claim based? Reference, please.

Let me point out again, as I have done here and elsewhere many times, that DSR made a definitive predictions (those are even the words used in the abstract) that electrons have substructure at a level below the level of detectability when the prediction was made, but high above the current level of detectability. DSR has thus been ruled out since a definitive prediction has been falsified. So, why do you keep touting it. Can you explain how it is still viable without epicycles?

As much as one would like to try, you cannot get away from the geometrical underpinnings that are taking place. Even current experimental data from planck2013 is to suggest? " Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results."?

Such expressions then seeks to define, while the geometric remains intact? B modes, are just another way in which to explain WMAP data. "Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive."

The topology of energy expression has not changed and just moves forward under genus development expressions. Say exactly why, the difficulties arise would affect this?

The fact that a single data sethas been able to expose three new problems is a tribute to thequality of the experiment and serious trouble for the paradigm,for the only reasonable conclusion from Planck2013 is that theanswer to Question B for inflationary cosmology is negativeInflationary paradigm in trouble after Planck2013

As Helbig well knows, I have extensively commented on and published papers on Discrete Scale Relativity's predictions for the dark matter and the presently undetectable halo surrounding the virtually singular electron.

Helbig ignores my scientific arguments and lies about DSR being falsified. DSR has not been observationally falsified to date.

In fact, I like the preliminary evidence in favor of DSR's predictions regarding the dark matter and the electron's structure. Search on "Predictions of Discrete Scale Relativity" for the true situation on 15 definitive predictions of DSR.

The fact that a pseudo-scientist like Helbig insists that I am wrong is actually encouraging since people like him are "often wrong but never in doubt."

I quote from http://adsabs.harvard.edu/abs/1987ApJ...322...34O which contains DSR's definitive prediction in the abstract:

"Two definitive predictions are also pointed out: (1)the model predicts that the electron will be found to have structure with radius $4\times 10^{-17}$ cm"

Modern accelerator experiments can compare QED predictions to observations, with the predictions treating electrons as point particles. They should see this structure if DSR were true, but they don't, so it isn't. (Maybe it's hidden behind an epicycle.)

Helbig can "rest [his] case" but as I have told him at least 4 times before the paper says more than the one sentence he quotes. It clearly identifies two basic possible structures for the electron. Subsequent developments favor the one Helbig conveniently and dishonestly fails to mention. Helbig has been made fully aware of DSR's empirically viable position on this issue. The man is a liar if he thinks or speaks otherwise.

Bottom line: Discussing science with Helbig is like discussing science with a Jehovah's Witness.

@RLO: If the paper discusses another possibility, then the abstract is misleading if it mentions just one possibility and calls it a definitive prediction.

You yourself have said many times that a theory which makes multiple predictions is not useful. I agree.

Searching some obvious places on the web will reveal that someone made a public retraction after I threatened him with legal action. His crime was different, though no less dishonest.

Note that I discuss science. You resort to name-calling. This will be my last public reply to you, here or anywhere else. You will probably say that your rhetoric defeated me, but I'm sure readers will draw the correct conclusion. Anyone reading your various comments and the replies to them will conclude that you are not interested in real scientific discussion, but rather in touting your theory which has been disproved according to your own criteria (not that anyone other than you ever took it seriously anyway) and personally insulting your opponents with lies.

I have just read the two papers by Steinhard and I must say that I share his concerns.

Another thing that worries me is that the running couplings seem to meet where inflation is supposed to have happened. (If this picture is right): http://i.stack.imgur.com/6qQn8.gif

To my taste this looks like a phase transition, where some unknown GUT symmetry is broken. This I think is what needs to be understood in the first place. Using the Friedmann equations and playing around with potentials to me looks more like alchemy.

Moreover the CMB spectrum exhibits scale invariance. What is the reason for this ? All I know is that phase transitions exhibit this feature.

Here is an interesting discussion on the topic:http://www.youtube.com/watch?v=beQ9fZ0jVdE

Would you suggest that a Platonist at heart is a alchemist?:)Would you not agree there is indeed a progression in the geometry(ordering of the geometries) that places the basis of thought schematics as a foundation in which experiments are conducive to such orientations?

Helbig is wrong again even though I patiently explained this to him 4 times before.

Discrete Scale Relativity still predicts electron structure with a radius of about 4 x 10^-17 cm.

I also pointed out to the selectively forgetful gasbag that when DSR scaling is applied to the Einstein-Maxwell equations (Kerr-Newman metric), then the electron can be shown to unambiguously NOT possess an event horizon at the Schwarschild radius (whereas the proton unambiguously does). It is virtually a naked singularity. This is the other possibility mentioned in the 1987 paper. But the electron is not quite a totally naked singularity!

DSR definitively predicts that the nearly singular electron has an extremely low-density envelope of subquantum scale particles. The radius of this envelope is about the Schwarschild radius. Someday we may be able to detect this envelope experimentally. There are already some hints of subquantum scale particles with sizes matching DSR predictions.

I have told all this to Helbig 4 or 5 times before. He is either demented, is losing his memory, or is an outright liar.

String Theory is interested in Condense Matter Theory which is what Laughlin represents.

I have had a picture manifesting in my head of a "cookie monster." Some may talk about spaghetti?:) I mean we could easily say that the cookie is like a "kernel of popcorn," but in this case the cookie is sort of like this.

The multiverse concept is a semantic nonsense, because it simply can never have observational support - at the moment, when something becomes observable, the it authomatically becomes a portion of our Universe.

The only motivation for multiverse is therefore ideological: the physicists do realize quite well, that the astronomers observed many things, which already violate the established theories - so that the proposal of multiverse would help them to save their credit, grants and salaries before the eyes of layman publics.

Now the question is, why the astronomers didn't propose an multiverse, when the geocentric model of solar system was violated with observation before five hundred years?

Plato,* Yes that's the pic, thanks for the improved link.* What I mean by alchemy is that in medieval times people desperately tried to "bake" gold but they of course failed because they used chemistry instead of "nuclear chemistry" - the wrong approach. I would not want to go that far to make a connection between Platonism and alchemy - too many great scientists are Platonists.* "String Theory is interested in Condensed Matter Theory". Yes, but I'm not shure if this is also so the other way around :-) At least this seems not to be the case for Subir Sachdev. He has written some very nice and readable articles, if you are interested. (E.g. http://sachdev.physics.harvard.edu/reviews.html)

The string theorists are particularly confused in this regard. We can ask easily, if the particles are formed with strings undulating in eight dimensions (which are belonging into our universe in this way) - why just these extradimensions don't belong into our Universe at the case of hyperdimensional branes of ekpyrotic cosmology?

MarcusM,I watched the youtube video you put up from Seti with Dr. Anthony Aguirre. I do not know if you agree with his analysis but I certainly don't.

At about 18:45 he says (paraphrasing) "What we measure is that the universe is uncurved." This is just incorrect. He also says at one point that the equations say the universe should get more curved as time goes on. But he is using the fact that dark energy is a complete mystery to him for his justification of everything he is saying.

My feeling is that he using eternal inflation and supersymmetry as a "given". So he is making up facts and ignoring the huge fact that dark energy means small acceleration now and small negative curvature now. He is also getting wrong the fact that homogeneity and isotropicness on large scales now implies directly high acceleration and high negative curvature at the beginning.

So to me he is just making a fool of himself. He is saying that dark energy is a complete mystery when it is not. He is also giving the wrong curvature of universe at various times and stating them as facts. And he is doing all this from an unreliable assumption coming from counter factual evidence that there is eternal inflation and that background is measurable as zero.

Wondering if you watched this weekend’s World Science Festival debate on the Multiverse hypothesis ? What this entailed was to pit Andrei Linde and Alan Guth against Andreas Albrecht and Neil Turok; with the former pair representing its advocates and the latter its detractors. In the end however I found it being difficult to take either side as being overly convincing, so I awarded this one to the thoughtfully witty moderator (referee), John Hockenberry, which I found as coming away as a 21st century Milton :-)

"From man or angel the great ArchitectDid wisely to conceal, and not divulgeHis secrets to be scanned by them who oughtRather admire; or if they list to tryConjecture, he his fabric of the heav'nsHath left to their disputes, perhaps to moveHis laughter at their quaint opinions wideHereafter, when they come to model heav'nAnd calculate the stars, how they will wieldThe mighty frame, how build, unbuild, contriveTo save appearances, how gird the sphereWith centric and eccentric scribbled o'er,Cycle and epicycle, orb in orb."

Hi Marcus,I appreciate your calm disposition on these debates. I confess I tend to get worked up but but I also try not to let it affect my actual logic.

I just find that the whole eternal inflation and multiple universe thing is not convincing. I think there is something in qft relating to a possible Hamiltonian over all time and all particles that is not being accounted for. If you assume there is a possible Hamiltonian then dark energy is simply the dividing point of kinetic and potential energy at different expansion times in the universe.

I've mentioned this before so if you read it before and I didn't convince you then it probably won't convince you now. I really think qft is incomplete in this accounting for energy globally. To me dark energy is the CC.

has any of these people EVER come up with an (1) original idea about nature that has (2) generated a definitive prediction, which (3) was verified experimentally?

I think you will find that the answer is a resounding "No!"

They have generated a lot of speculative pseudo-science (extra-dimensions, multiverse rubbish, supersymmetry hype, anthropic pretzel logic, etc., etc., but not one verified discovery about nature.

What does this say about the state of theoretical physics today? What does this say about the credulous sycophants who hang on their every fatuous pronouncement? What does this say about the science journalism that enables this hype?

And finally, where have you gone Albert Einstein? Science turns its lonely and very disappointed eyes to you.

Eric"I just find that the whole eternal inflation and multiple universe thing is not convincing."That's what I am saying too. The only point that I wanted to make, and that's what got clear to me when I watched the video, is that if you buy inflation it's hard to avoid the conclusion that there is a multiverse.BUT I have done some readings on inflation in the meantime and in fact there are so many (often fundamentally) different models of inflation, that the word actually is quite ambiguous (the same applies to THE multiverse).What concerns DE, I have a very beautiful theory but I know that there are thousands of papers on the topic suggesting to me that any theory at the moment is experimentally hopelessly underconstrained.